Inkjet printing system is a printing system comprising directly jetting ink droplets from very fine nozzles, and depositing the ink droplets on a recording medium to give characters and images. This system has advantages that not only the used device shows a low level of noise and has excellent operability, but also the coloration is facilitated and plain paper can be used as a recording medium. Therefore, this system has been widely used in the recent years.
In an ink used for inkjet printers, water-soluble dyes and polyhydric alcohols are used in order to prevent the ink from being clogged in the nozzles. However, this ink is poor in water resistance and light fastness. Especially, when the ink is used as an ink for thermal jet system, there arise defects such that the dye is oxidized due to the heat of a heater surface, and that the ink is easily scorched on the heater surface, thereby lowering the discharging ability.
In order to eliminate this defect, there has been proposed an ink comprising a water dispersion of a vinyl polymer having a specified structure in which a hydrophobic dye is incorporated (Japanese Patent Laid-Open Nos. Hei 9-241565 and Hei 9-286939). However, this ink has defects that light fastness and printing density are insufficient because dye is used.
An object of the present invention is to provide a water-based ink for inkjet printing which is excellent in light fastness, imparts high printing density, gives a recording medium no distortions, and is excellent in water resistance, rub resistance, and highlighter-fastness.